It is well-known that water, oil and gas can be found in a reservoir rock. It is important to know how these three fluids are distributed in the various points of the reservoir in order to determine the quantities of hydrocarbons in place and for production forecasts. The water present in hydrocarbon zones is referred to as interstitial or irreducible water.
The quantity of oil in place is estimated from the following relation:
Oil in place=Rock volume*porosity*(1-Scwo)/FVF
where Scwo is the interstitial water saturation and FVF the formation volume factor which is equal to the volume ratio between bottomhole conditions and standard conditions.
Knowledge of the interstitial water saturation can be obtained through the following various measurements:
analysis of the wireline logs obtained by the induction or resistivity sondes lowered into a well crossing the reservoir rock, PA0 analysis of the petrophysical measurements performed most often on one or more reservoir rock samples. The water saturation is calculated at the laboratory from capillary pressure curves.
Water saturations calculated from logs and correlated with petrophysical measurements on a rock sample are thereafter used in calculations, particularly in reservoir models. The interstitial water saturation values thus calculated have been kept constant until now during reservoir simulation model studies.